A normal optical microscope needs at least two additional components to perform polarized light microscopy. For detection of birefringence it is necessary to use linearly polarized light for illumination. Therefore, two polarizing filters have to be inserted in the beam path of the microscope. The first polarizing filter produces the polarized light to illuminate the specimen and the second polarizing filter, called the analyzer, restricts detected light to refracted light.
The polarizing filters have to be at an angle of 90° to each other to achieve the so called “dark position”. When the polarizing filters are set at this position, no light will pass to the camera or eyepieces and the image will be dark. Setting up the dark position is an important step for polarized light microscopy as it ensures that only light which experienced a change in the polarization plane due to the specimen will be visible.